Heated humidity sensors are hereby described that allow the measurement of humidity in gases having a dewpoint above the temperature of the surrounding environment. Problems associated with such gas streams include lack of sensor accuracy at high relative humidity and the possibility of liquid condensate flowing onto the sensor. A further disadvantage of measurement in high dewpoint gases is the possibility of sensor failure or misreading.
Such humidity sensors could be used, for example, as part of a humidity-controlled medical humidifier. This introduces design constraints such as small size, robustness and ability to be sterilised to prevent cross-contamination between patients.
When a medical humidifier is used with a respiratory ventilator, gas flow and absolute humidity can change rapidly in a cyclic manner. These changes typically happen faster than the response time of the humidity sensor, which gives a “time average” of the humidity surrounding it. This means that certain key humidity parameters, such as average absolute humidity during the inspiratory part of the flow cycle, are unable to be measured.
In U.S. Pat. No. 4,143,177 capacitive-type humidity sensor is described, as well as how a heater and temperature sensor can be incorporated into the humidity sensor to allow stable temperature control of the humidity sensor at temperatures above that of the surrounding gas. This enables the humidity sensor to avoid condensation in gases of high humidity. The heater can be manufactured around or underneath the humidity sensor. It also describes how the heater element can be simultaneously used to measure temperature, i.e. the heater element and temperature sensor are combined in one element. This makes sensor construction easier.
U.S. Pat. No. 5,777,206 also describes a heated capacitive-type humidity sensor. A single resistor is used in this patent both as a heater and a temperature sensor, for controlling the humidity sensor temperature above the gas temperature. U.S. Pat. No. 5,777,206 also describes calculating the absolute or relative humidity from knowledge of the temperature of the sensor. Further disclosure includes a method for determining the gas flow rate based on the amount of heat being supplied to the heated capacitive sensor to determine the gas flow rate past the sensor. Whereas U.S. Pat. No. 5,777,206 uses a resistor to provide sensor heating and temperature measurement, U.S. Pat. No. 4,143,177 uses a P-N semiconductor diode junction to provide the same functions.